<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>圆角练习</title>
    <style>
        .c1{
            width: 200px;
            height: 200px;
            border-radius: 50%;
            border-top: 100px solid red;
            border-right: 100px solid green;
            border-bottom: 100px solid blue;
            border-left: 100px solid yellow;
            margin: 100px auto;
        }
    </style>
</head>
<body>
<div class="c1"></div>
</body>
</html>